The analysis of isotope ratios is widely used for the investigation of atmospheric trace gases. We measured the stable isotopes d13C and dD of atmospheric CH4 as well as d13C, d18O and the 14CO concentrations of atmospheric CO at Izaña, Tenerife, from 1996
to 1999. We report the rst directly measured dD seasonality for atmospheric background CH4. The large seasonal cycles in mixing and isotopic ratios of CH4 and CO enable inferences about the underlying source and sink processes.